When downlink Coordinated MultiPoint (DL CoMP) transmission is used, such as in a Long-Term Evolution (LTE) wireless communication network, downlink signals and channels received by a UE might come from different transmission points.
FIG. 1 illustrates a CoMP scenario, in which UE 3 and UE 1 are macro UEs, and are thus synchronized to a macro cell denoted as Macro eNB in terms of time and frequency via a Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), and/or Cell-Specific Reference Signals (CRS). However, UE 3 is decoding a signal sent by a pico cell denoted RRH1 (Remote Radio Head 1), such as a Physical Downlink Shared Channel (PDSCH) and/or an enhanced Physical Downlink Control Channel (ePDCCH). Since the macro cell is further away from the pico cell, UE 3 will observe a time difference between received signals. If there is some frequency difference between oscillators in the macro cell and the pico cell, or if UE 3 is moving in such a way that distances to the two transmitters are changing at different rates, then a phase ramp in the time and frequency domain is created. The resultant phase ramp can severely degrade the demodulation performance of the receiver in this CoMP scenario. Although the pico cell is denoted as RRH1 in FIG. 1, it may be denoted as any other particular type or configuration of radio nodes in a wireless communication network.
To address this problem, related work has started in RAN1 and RAN4 working groups of the 3rd-Generation Partnership Project (3GPP), under a working topic named “quasi-colocation,” where the term “quasi-colocation” refers to a situation where two transmitters may be deemed to be located at the same point (collocated), for purposes of evaluating a particular property of the signals transmitted from those transmitters. To be clear, two transmitters may be deemed to be quasi-colocated for one property, such as frequency shift, while not deemed to be quasi-colocated for another, such as delay spread. To date, RAN 1 has defined sets of reference signal ports that can be considered as quasi-colocated or not, while RAN 4 is continuing to investigate the impacts on receiver performance when the non-colocated/non-quasi-colocated assumption is considered.
For demodulation of PDSCH, Table 1 summarizes assumptions that may be applied, as agreed in RAN1 meeting #70.
TABLE 1Agreements from RAN1 meeting #70 for PDSCHCRSCSI-RSPDSCH DMRSMay be assumed as quasiWithin a CSI-RS resource, CSI-RS portsMay be assumed as quasico-located wrt all longmay be assumed as quasi co-located wrtco-located within aterm channel properties{delay spread, rx power, frequency shift,subframe wrt to {delay{delay spread, rx power,Doppler spread, Received timing}.spread, rx power,frequency shift, DopplerBetween CSI-RS resources CSI-RS portsfrequency shift, Dopplerspread, Received timing}shall not be assumed as quasi co-locatedspread, Received timing}within the serving cell.wrt {delay spread, rx power, frequencyshift, Doppler spread, Received timing}.CRS-RSPDSCH DMRSPSS/SSSCRSBehaviour A: CRS, CSI-RSBehaviour A: CRS, CSI-RS andPSS/SSS andand PDSCH DMRS may bePDSCH DMRS may be assumed asCRS ports forassumed as quasi co-ocatedquasi co-located wrt {frequency shift,a serving cellwrt {frequency shift,Doppler spread, Received timing,may beDoppler spread, Receiveddelay spread)assumed astiming, delay spread}Behaviour B: CRS and PDSCH DMSquasiBehaviour B: CRS andshall not be assumed as quasico-located wrtCSI-RS shall not be assumedco-located wrt {frequency shift,{frequencyas quasi co-located wrtDoppler spread, Received timing,shift,{frequency shift, Dopplerdelay spread}Receivedspread, Received timing,timing}.delay spread}CSI-RSBehaviour A; CRS, CSI-RS andPDSCH DMRS may be assumed asquasi co-located wrt {frequency shift,Doppler spread, Received timing,delay spread}Behaviour B: PDSCH DMRS and aCSI-RS resource indicated byphysical layer signaling may beassumed as quasi co-located wrt{frequency shift, Doppler spread,Received timing, delay spread}
As indicated by Table 1, the UE can be in either of two different states, Behavior A and Behavior B. For Behavior A, all reference signal (RS) ports may be assumed as quasi-colocated with one another, with respect to all properties (frequency shift, Doppler spread, received timing, and delay spread). For Behavior B, on the other hand, the UE may assume only that DeModulation-Reference Signal (DM-RS) is quasi-colocated with Channel-State Information Reference Signal (CSI-RS), and not with Cell-specific Reference Signal (CRS).
The notion that the UE may assume that an RS port A is quasi-colocated with an RS port B with respect to a channel property X means that the UE is allowed to use RS port A and B in any combination (e.g., only RS port A, only RS port B, or both together) to determine channel property X. Transmission modes 1-9, as defined by the LTE specifications, are all only using Behavior A.
Further agreements with respect to quasi-colocation were made in the RAN1 #71 meeting. It was agreed that whether transmission mode 10 uses Behavior A or Behavior B at a given time can be configurable, using Radio Resource Control (RRC) signaling. Further, the relation between CRS and CSI-RS was changed in Behavior B, so that CRS and CSI-RS may be assumed quasi-colocated with respect to Doppler shift and Doppler spread.
Summarizing the RAN1 #71 agreements:                For Behavior B:                    For each CSI-RS resource, the network shall indicate by RRC signaling that CSI-RS ports and CRS ports of a cell may be assumed as quasi co-located with respect to the following properties                            {Doppler shift, Doppler Spread}                RRC signaling includes:                                    Cell ID for quasi-colocated CRS                    Number of CRS ports                    Multicast-Broadcast Single Frequency Network (MBSFN) configuration                    Signaling details are left up to RAN2                                                                                
Note that the notion of a Doppler shift includes all effects that create a frequency shift in the received signal. For example, if two transmission points have clocks that run at (slightly) different rates, that will be seen on the UE side as a Doppler shift between the received signals corresponding to the two points. Similar reasoning applies for when a transmission point and the UE are using clocks with different rates. The term “point” may be used in the description that follows to refer to a transmission point. Naturally, movement of the UE relative to the transmission points may also create a Doppler shift. Thus, both clock rate differences and UE mobility contribute to the overall Doppler shift seen on the UE side. In general, the description that follows refers to a frequency offset, without distinguishing between the cause or causes of the frequency offset, and it should be kept in mind that from a UE perspective this is the same thing as a Doppler shift.
More particularly, the description that follows focuses on the frequency offset between PSS/SSS/CRS ports and DM-RS ports, in an LTE system, and elaborates on the estimation of such a frequency offset. Before proceeding further, however, some background information about the frequency offset is introduced.
FIG. 2 illustrates frequency offset generation due to clock rate differences, and its impact on a subcarrier on the UE side.
As shown in FIG. 2, the UE on the right-hand side of the figure is synchronized in frequency to TP#0, but is decoding data/control channels sent by TP#1. A frequency difference of Δf  Hz exits between the two transmission points (TPs), due to slightly different clock rates. This frequency offset creates an extra and undesirable phase ramp (phase rotation) in the receiver after Fast-Fourier Transform (FFT), as illustrated in FIG. 2.
The resultant phase ramp in time caused by the frequency offset can severely impact the demodulation performance of CoMP.
To illustrate this, simulation results from a CoMP system is presented FIG. 3, where a DM-RS based PDSCH is transmitted from one point while the serving cell CRS is transmitted from another point with a different frequency offset. The simulations in FIG. 3, as well as all other simulations illustrated in the present disclosure, were carried out under the following assumptions: Frequency-Division Duplexing (FDD); 1 UE; two TPs; 10 MHz system bandwidth; Extended Pedestrian A (EPA) channel; 5-Hz Doppler; 2×2; Transmission Mode 9 (TM9); 2 CRS ports; 2 CSI-RS ports; 1 DM-RS port; 50 RB allocated to PDSCH; fixed Modulation and Coding Scheme (MCS) of 64 QAM at ¾ coding rate; and the UE and the two TPs are perfectly synchronized in time.
Specifically, FIG. 3 illustrates a decline in throughput with increasing frequency offset without estimating and compensating for the frequency offset between DM-RS ports and CRS ports. As shown in FIG. 3, the demodulation performance of the PDSCH is severely degraded by the frequency offset, if the UE does not do anything to compensate for the phase ramp.